1. Field of the Invention
The present invention relates to a high voltage AC power supply circuit adapted for use in an image forming apparatus such as copying machine, printer or the like.
2. Related Background Art
In the conventional high-voltage AC power supply circuit for an image forming apparatus, a DC-DC converter circuit is inserted between a low voltage terminal and a ground terminal of a secondary coil of an AC high-voltage transformer in order to control the DC current in an AC charging wire of a separating charger constituting a load. Particularly if the control range of the DC current becomes wider and spreads from the positive to the negative side, there are required a DC-DC converter with a positive output and another DC-DC converter with a negative output.
FIG. 4 schematically show the basic structure of the above-mentioned conventional example, wherein shown are an AC high-voltage power source 21 including an AC high-voltage transformer; a DC-DC converter 22; an output terminal P1; a separating charger 23 constituting a load; and a direct current i.sub.dc required for an electrostatic sheet separation. The direct current i.sub.dc has to be in a direction opposite to the output of the DC-DC converter 22.
FIG. 5 is a circuit diagram of the DC-DC converter 22, wherein the primary DC input to a transformer T31 is chopped by a switching transistor TR31 controlled by an output control circuit 31, and the generated secondary output is rectified and smoothed by a diode D31 and a capacitor C31.The secondary rectifying circuit is further provided with a bleeder resistor R31.
FIG. 6 shows the voltage-current characteristic of the DC-DC converter 22, indicating the DC voltage v.sub.dc and the direct current i.sub.dc respectively in the ordinate and in the abscissa. FIG. 7 shows the voltage-current characteristic of the separating charger 23 receiving power supply from said high voltage AC power supply circuit, indicating the current i and voltage v respectively in the ordinate and in the abscissa. As shown in FIG. 7, the separating charger 23 has an apparently asymmetrical characteristic on the positive and negative sides, accepting more easily a current of the negative side.
In the above-explained structure and characteristics, the resistance of the breeder resistor R31 has to be made infinitely small, in order to bring the direct current i.sub.dc to -400 .mu.A. For this reason the DC-DC converter 22 has to be of a high power type, which gives ride to the drawbacks of heat generation and a high cost.
If the control range of the direct current i.sub.dc is further spread from the limit of about zero in FIG. 6 to a positive area, it becomes necessary to serially insert a DC-DC converter 25 of a fixed output, with a polarity opposite to that of the variable-output DC-DC converter 22, as shown in FIG. 8. Since the control range of the direct current i.sub.dc on the load side becomes narrower by the output voltage of the fixed-output DC-DC converter 25, the output control range of the variable-output DC-DC converter 22 has to be accordingly widened, so that the power loss becomes even larger.
As explained in the foregoing, the DC-DC converter 22 requires a small resistance in the bleeder resistor R31 shown in FIG. 5, thus resulting in a significant power loss when the control range of the direct current i.sub.dc required for electrostatic sheet separation becomes wider. Also it requires a complex circuit structure.